According to a first prior art, a Near Field Communication (NFC) reader periodically checks for available nearby tags by sequentially polling for all compatible standards and waits for tag response. In a mobile device, this method will significantly drain battery of the NFC reader as, for each standard, a possible tag must be given time to respond, which means notably at least 5 ms for field powering, during which the reader must send its full power output for enabling the tag to load modulate. This results in a high duty cycle for the polling time with full transmission power causing high average power consumption in the NFC reader, for detecting possible present tag.
According to a second prior art, it is known a method called Low Power Tag Detection (LPTD) which is used by the Near Field Communication reader to detect the presence of a nearby tag. The method is based on a chirp stimulus that is used to measure, on-chip, the resonance frequency and the quality factor at the output of the reader transmitter. According to the respective values of the measured resonance frequency and quality factor, the reader detects if a nearby tag is present or not in its vicinity. This second prior art detects tag presence or absence by observing the detuning of the antenna of the reader transmitter, that a possibly present tag represents, to the reader front-end.
However, this Low Power Tag Detection method suffers from fault tag detection, as will be illustrated notably by FIGS. 1a and 1b. An antenna coupling between reader and tag is simulated. The reader antenna quality factor is reduced with series resistors, and matched to the reader transmitter pins via a three-capacitor matching circuit. The reader transmitter represents a low impedance (voltage) drive to the reader transmitter pins, while also replicating the reader transmitter current waveform into the reader receiver for detecting load modulation and nearby tag presence if any.
According to this Low Power Tag Detection method, every couple of hundreds of milliseconds, the reader seeks for a nearby tag. To seek for a nearby tag, the reader transmitter sends out a chirp signal. In the case of the NFC IP of CG2910, the chirp signal frequency is swept from 12 MHz to 15 MHz. In the absence of a nearby tag, due to the reader load across the reader transmitter output terminals, the chirp signal resonates at the resonance frequency of the reader (for example 13.56 MHz for the NFC IP of CG2910). However, in the presence of a nearby tag, the load across the reader transmitter output terminals is impacted by the presence of the tag, which implies that the chirp signal resonates at a frequency different from the reader resonance frequency (for example 13.56 MHz for the NFC IP of CG2910).
Since the reader receiver is used to copy the reader transmitter output signal (chirp), this copy is then analyzed to calculate the value of the resonance frequency of the chirp signal. Once a different resonance frequency is detected, meaning a nearby tag is detected, the reader starts a new NFC communication with this newly detected nearby tag. This is how this Low Power Tag Detection method works.
FIG. 1a shows an example of detuning of an antenna of a reader due to a nearby tag, when reader and nearby tag both resonate at the same frequency. This common resonance frequency is 13.56 MHz. The amplitude A of the reader transmitter current is expressed in decibels dB and plotted as a function of the frequency expressed in Mega Hertz MHz. The detuning of the resonance frequency of the reader, from its original value, is all the more important that the nearby tag becomes closer to the reader and that the coupling factor k increases, what is shown through the multiple curves plotted on FIG. 1a. The corresponding respective values of the coupling factor k are 0-2-4-6-8-10%, the higher peak curves corresponding to the lower coupling factor k values. In this example, the reader quality factor value is 25, the tag quality factor value is 35, and the reader matching circuit comprising two series capacitors of same capacitive value 2*C1r, a capacitor in parallel of capacitive value C2r, there is the following relation: C1r/(C1r+C2r)=0.9, the current itx circulating in the reader matching circuit and in the reader antenna has a value of 100 mA rms.
FIG. 1b shows an example of detuning of an antenna of a reader due to a nearby tag, when reader and nearby tag each resonate at a different frequency. FIG. 1b is quite similar to FIG. 1a, except that the resonance frequency of the reader is 13.56 MHz, whereas the resonance frequency of the tag is 16 MHz. The amplitude A of the reader transmitter current is expressed in decibels dB and plotted as a function of the frequency expressed in Mega Hertz MHz. The detuning of the resonance frequency of the reader, from its original value, is all the more important that the nearby tag becomes closer to the reader and that the coupling factor k increases, what is shown through the multiple curves plotted on FIG. 1b. The corresponding respective values of the coupling factor k are 0-2-4-6-8-10%, the higher peak curves corresponding to the lower coupling factor k values. In this example, the reader quality factor value is 25, the tag quality factor value is 35.
The Low Power Tag Detection method according to second prior art presents weakness and ineffectiveness in the presence of a nearby ground plane or any metallic or magnetic or lossy body. This disadvantage of Low Power Tag Detection method according to second prior art, is that a nearby tag is not the only possible cause for detuning. Through measurements on the same reader as the one used for FIGS. 1a and 1b, it was possible to prove that nearby metallic, magnetic, or lossy objects will also detune the reader transmitter circuit response, as can be seen on FIGS. 2 and 3. This disadvantage is a major one, since for example in NFC mobile applications, there will be a metallic ground plane on the host platform PCB (“printed circuit board”), and the battery will have an effect similar to a metallic plate too.
FIG. 2 shows an example of detuning of an antenna of a reader due to a nearby metal plane. The amplitude A of the reader transmitter current is expressed in decibels dB and plotted as a function of the frequency expressed in Mega Hertz MHz. When an NFC antenna coil is close to a metallic object, the generated magnetic field and the antenna inductance and quality factor will change due to induced circulation currents. The reader transmitter current is respectively measured with a parallel metallic plane at 3, 5, 10, 15, 20 and 25 mm distance. FIG. 2 shows how the front-end of the reader progressively detunes because of a metallic surface coming closer to the reader. The metallic object has a severe impact on the resonance frequency of the reader, and this will most probably trigger a false tag detection during processing of the Low Power Tag detection method according to the second prior art. Its lack of detection reliability in case of metallic surface in the vicinity of the reader which mistakes it for a tag, is a first major disadvantage of this second prior art.
FIG. 3 shows an example of detuning of an antenna of a reader due to a nearby lossy plane. The amplitude A of the reader transmitter current is expressed in decibels dB and plotted as a function of the frequency expressed in Mega Hertz MHz. The nearby lossy plane comes closer to the reader in the same conditions as the metallic surface in FIG. 2. FIG. 3 shows how the front-end of the reader progressively detunes because of a lossy plane coming closer to the reader. The lossy object has a severe amplitude impact on the peak of the reader transmitter current, and this will most probably trigger a false tag detection during processing of the Low Power Tag detection method according to the second prior art. The lack of detection reliability, of the second prior art, in case of lossy object in the vicinity of the reader which mistakes it for a tag, is a second major disadvantage.